How gravity and continuity in UT1 moved the Greenwich meridian

Malys, Stephen; Seago, John H.; Pavlis, Nikolaos K.; Seidelmann, P. Kenneth; Kaplan, George

In 1884, the International Meridian Conference recommended that the prime meridian “to be employed as a common zero of longitude and standard of time-reckoning throughout the globe” pass through the “centre of the transit instrument at the Observatory of Greenwich”. Today, tour-ists visiting its meridian line must walk east approximately 102 meters before their satellite-navigation receivers indicate zero longitude. This offset can be accounted for by the difference between astronomical and geodetic coordinates — deflection of the vertical — in the east-west direction at Greenwich, and the imposed condition of continuity in astronomical time. The coor-dinates of satellite-navigation receivers are provided in reference frames that are related to the geocentric reference frame introduced by the Bureau International de l'Heure (BIH) in 1984. This BIH Terrestrial System (BTS 84) provided the basis for orientation of subsequent geocentric reference frames, including all realizations of the World Geodetic System 1984 (WGS 84) and the International Terrestrial Reference Frame (ITRF). Despite the lateral offset of the original and current zero-longitude lines at Greenwich, the orientation of the meridian plane used to measure Universal Time has remained essentially unchanged.

Return to invited speaker list